Tripartite developer for electrostatic images



United States TRIPARTITE DEVELOPER FOR ELECTROSTATIC IMAGES No Drawing.Application December 23, 1954 Serial No. 477,380

10 Claims. (Cl. 25262.1)

This invention relates to improved mixtures for developing electrostaticand electrophotographic images, and more particularly to a tripartitedeveloper mixture adapted to enhance the reproduction of half tone,continuous tone and other dark-area copy by cascade development.

In the electrophotographic process, the photoconductive layer of axerographic plate is first charged electrostatically, the chargedsurface then being exposed under light to the subject to be copied,thereby forming a latent electrostatic image. The latent image isdeveloped by subjecting the image layer to a cascading operation ofdeveloper powder to produce a powder image, which is thereaftertransferred and aflixed to paper or other transfer medium.

Commercial developer materials are presently known which consist of twomajor components; namely, electroscopic powder in combination with acarrier. Developers of this type are constituted by a dry mixture ofloose, movable particles of finely divided electrostatically-attractableand pigmented powder, generally called toner powder, and separategranular carrier material. The granular material is composed of looseparticles, each of which includes a core, such as a glass bead, which isencased in a triboelectric resinous coating. To effect adherence of theresinous casing to the core, an intermediate bonding layer, for example,a liquid resin which contains a hydrochloric acid catalyst, is firstapplied to the core particles.

The casing and powder particles have a triboelectric relationship ofopposing polarity, the powder particles thereby being electrostaticallycharged through triboelectric action by mixing with the granular carriermaterial to adhere movably to the surface of the carrier granules.Hence, the powder particles are attractable by anelectrostatically-charged insulating layer; whereas the casing of thecarrier which is correspondingly charged to opposite polarity, isadapted to attract the charged powder and to remove them from unchargedareas of the electrostatic latent image surface when cascadedthereacross. Suitable two-component developer mixtures are more fullydisclosed in the Patents 2,618,551 and 2,618,552, issued on November 18,1952, and in Patent 2,638,416, issuedMay 12, 1953.

5 When making use of conventional two-component developers in normalcascade development, it is found that large dark areas are fullydeveloped near their edges but that the development is quite inadequateat a distance of inch or so in from their edges. This phenomenon, whichis called the halo eifect, arises from the fact that the electricalfield immediately above the plate is intense only in those areas wherethere is a gradient of electrical potential along the surface of thexerographic plate. However, in large dark areas, where there is littleor no potential gradient along the surface of the plate, the strength ofthe electrical field which attracts toner particles is practically zero.

Another drawback encountered during development with commercialdeveloper mixtures is the adhesion of carrier or developer powder to thexerographic plate. This adhesion is customarily referred to as blocking,which blocking action interferes with proper development. Whencommercial developer is used with conventional xerographic machines, itbecomes necessary to overcome blocking by tapping the plate sharply todislodge any carrier which clings to the plate after development.

Accordingly, it is the primary object of the present invention toprovide a novel and improved developer, whereby large dark areas, halftones and continuous-tone images are more completely developed in thecascading development process. It is a further object of the inventionto provide a developer which minimizes blocking effects.

More particularly, it is an object of the invention to provide atripartite developer constituted by a conventional two-componentdeveloper in conjunction with an additive which imparts superiorperformance characteristics to the resultant mixture. Generally stated,existing xerographic developers are rendered more effective whenmodified by the incorporation of certain electrically conductiveadditives. Iron, cobaltic oxide, stannous oxide, zinc and ferromauganesein powdered form are among the most effective additives discovered.

A preferred form of toner for use in conjunction with an additive inaccordance with the invention is that known commercially as the A-ltoner. A toner of this type consists of particles of pigmenting materialencased in or surrounded by an insulation material which acquires bycontact with the granular carrier material an electrostatic chargehaving a polarity opposite to that acquired by the granular material andopposite to that of the photoconductive insulating layer of the plate.This toner is in the particle size range of from 0.1 to 20 microns andthe granular carrier material is in the size range of from 30 to 200mesh.

The coloring material may be carbon or other suitable pigments and theinsulating material may be a rosinmodified phenol-formaldehyde resin,such as known commercially as Amberol F-71, manufactured by Rohm & HaasCompany, The Resinous Products Division, Washington Square, Philadelphia5, Pennsylvania, or asphaltum, or other suitable material.

The pigmented electroscopic powder is prepared by first micronizing theresin material, such as Amberol P -71, after which it is mixed withapproximately 5% by weight of carbon black or other pigmenting materialand the mixture ball-milled for about four hours in a ceramic jar withstone pellets. The mixture is then heated to a temperature of about 300F. or to flowing viscosity and mixed for five minutes in order to encasethe pigmenting particles with the Amberol F71. The mass is thenpermitted to cool, after which it is broken into small chunks and againmicronized.

The pigmented electroscopic powder is then in condition for mixing witha granular carrier such as polymerized methyl methacrylate, having amelting point of approximately 257 F., known commercially as Lucite andmanufactured by E. I. du Pont de Nemours & Company, Wilmington,Delaware, of other material either conducting or insulating, providedthe particles of granular material when brought in close contact withthe electroscopic powder particles acquire a charge having an oppositepolarity to that of the electroscopic,

powder particles, such that the electroscopic powder particles adhere toand surround the granular carrier particles. The granular carriermaterial is selected so that the particles acquire a charge having thesame polarity as that of the photoconductive insulating layer of theplate on which the electrostatic image is produced, and an electricalattractionfor the electroscopic powder particles considerably less thanthat of the charged areas of the plate and somewhat greater than thedischarged areas of the plate.

It has been discovered that the presence of a small quantity of one ofseveral electrically-conductive powders to a standard toner, such as A-ltoner, modifies the characteristics of the developer combinationwherebylarge dark areas, half tone and continuous tone images are much morecompletely developed in cascade operation than in the absence of theadditive. Moreover, it has been found that the blocking action whichaccompanics the use of existing developers is minimized by theincorporation of an additive therein; hence, the need to tap the platesharply to remove particles adhering thereto is obviated orsubstantially reduced.

The following additives in the powdered state have been found to affordsubstantial benefits when mixed with a standard two-component developerin the ratios hereinafter disclosed:

1. Iron 2. Cobaltic oxide Stannous oxide Zinc . Ferromanganese CopperCupric carbonate, basic (Bakers Analyzed Chemical Reagent, lot 81045) 8.Zinc carbonate 9. Manganese carbonate Cupric oxide Lead acetateZirconium Nickel carbonate The above-listed materials are all effectiveadditives and give nearly the same results. Any selection from thesematerials must therefore be based on other considerations, such asavailability, cost and toxicity.

It has been determined that the optimum relationship of the threecomponents constituting the developer mixture is as follows: 75 grams ofcarrier material, 0.6 gram of toner, and 1.3 grams of powdered additive.The volumetric equivalent of this relationship is five parts of toner toone part of additive powder. The additive particles are all in powderform and are preferably of a size that will pass through a 270 meshscreen. The selected additive is mixed thoroughly with the standarddeveloper to form the tripartite developer in accordance with theinvention.

A lesser ratio of additive to toner may also be used with beneficialresults; however, when this ratio is reduced to one part additive to tenparts of toner, the results are appreciably poorer with respect to darkarea coverage than with the above-defined optimum relationship. On theother hand, the use of more additive than established by optimumrelationship will also alford good dark area coverage, but when theratio is raised to a point at which there is one part additive to threeparts toner, dark streaks are produced on the background areas of theimage. too much additive causes excessive deposition of toner inbackground areas, while too little additive lessens the desired effect.

All of the above-listed additives possess the following commonproperties:

First, they are good electrical conductors, as compared to materialswhich are ineffective as additives. For example, the resistivity(ohms-centimeterx of five of the above-listed additives in powdered formis as follows: Iron (2.20), stannous oxide (0.14), zinc (12.7),ferromang'anese (142), copper (12.7). On the other hand, many of thematerials which are inefiec- It will be evident therefore that tive asan additive have a relatively high resistivity. For example, magnesiumselenite which has a resistivity of l2,lO0.O 10 is ineffective as anadditive. Similarly, polyethylene in powder form has a resistivity of28,40O.O 10

Second, the triboelectric relationship of the abovelisted additives withrespect to the carrier of the developer as measured by an electrometeror similar means is strongly negative.

Third, the nature of the above-listed additives with respect to thetoner material is such as to resist or inhibit coating of the additiveparticles by toner particles. As presently understood, the theory whichaccounts for the effectiveness of the selected additives in improvingthe development of dark areas is based on the assumption that theadditive, which is electrically conductive and has a strongly negativetriboelectric relationship with respect to the carrier, functions bydischarging small randomly distributed portions oflarge dark areas, orby preventing carrier particles from rials (not listed supra) havingrelatively good conductivity fail to operate efiectively as an additivefor the purpose intended. For example, barium chloride, a Whitecrystalline powder of good conductivity, whenmixed with A-l toner andthereafter examined under a microscope will exhibit a coated appearance,the white particles of chloride being coated with fine particles oftoner. Silver chloride will exhibit the same coated appearance whensimilarly prepared.

It appears therefore that some materials, while good conductors, aresubject to a coating action by the toner particles, which action resultsin the formation of an insulating sheath effectively rendering theadditive nonconductive: on the other hand, the selected additives,listed supra, resist or inhibit a coating action and remain conductive,whereby they function to change the distribution of electrical charge onthe surface of the xerographic plate.

In summary, the additives in accordance with the invention are materialsin powder form (a) possessing relatively good electrical conductivity,(b) having a tribm electric relationship to the carrier component of thedeveloper which is strongly negative, and (c) exhibiting substantialfreedom from a coating action by the.

toner component of the developer, whereby the conductivitycharacteristic is maintained in the tripartite mixture.

While there have been disclosed specific additives for improving darkarea coverage in the development of electrostatic images, it is to beunderstood that other materials which possess the required combinationof properties fall within the scope of the invention, and it is intendedtherefore in the appended claims to cover all such materials.

What is claimed is:

1. A tripartite developer for electrostatic latent images comprising amixture of three components, said mixture comprising a first componentconstituted by loose, movable particles of electrostatically-attractabletoner powder, a second component constituted by granular carriermaterial composed of loose particles, each particle of said secondcomponent including a core and a coating bonded thereto and eachparticle of said second component having a triboelectr-ic relationshipof opposite polarity to the toner powder, and a third componentconstituted by a powder composed of particles having a resistivity inpowder form of no greater than.

range between said first and said second components, and said thirdcomponent being added to a proper xerographic developer mixture of saidfirst and said second component in the ratio range of from 1 part ofsaid third component to 3 to parts of said first component.

2. A developer in accordance with claim 1 in which said third componentis constituted by iron powder.

3. A tripartite developer for electrostatic latent images comprising amixture of three components said mixture comprising a first componentconstituted by loose, movable particles of electrostatically-attractabletoner powder, a second component constituted by granular carriermaterial composed of loose particles, each particle of said secondcomponent including a core and a coating bonded thereto and eachparticle of said second component having a triboelectric relationship ofopposite polarity to the powder of said first component, and a thirdcomponent constituted by a powder composed of particles having aresistivity in powder form of no greater than lead acetate, a stronglynegative triboelectric relationship to said carrier particles andexhibiting freedom from coating action relative to said toner particles,said first component compriisng particles in the size range of from 0.1to 20 microns, said second component comprising particles in the sizerange of from 30 to 200 mesh and said third component being of anintermediate size range between said first and said second components,and said third component being added to a proper xerographic developermixture of said first and said second component in the ratio range offrom 1 part of said third component to 3 to 10 parts of said firstcomponent.

4. A developer in accordance with claim 3 in which said third componentis constituted by cobaltic oxide.

5. A developer in accordance with claim 3 in which said third componentis constituted by stannous oxide.

6. A developer in accordance with claim 3 in which said third componentis constituted by zinc.

7. A developer in accordance with claim 3 in which said third componentis constituted by ferromanganese.

8. A developer for electrostatic latent images comprising a threecomponent mixture, the first component comprising xerographic tonerpowder, the second component comprising xerographic carrier particlesand the third component comprising an additive selected from the groupconsisting of iron, cobaltic oxide, stannous oxide, zinc,fer-romanganese, copper, basic cupric carbonate, zinc carbonate,manganese carbonate, cupric oxide, lead acetate, zirconium, and nickelcarbonate, said first component comprising particles in the size rangeof from 0.1 to 20 microns, said second component comprising particles inthe size range of from 30 to 200 mesh and said third component being ofan intermediate size range between said first and said secondcomponents, and said third being added to a proper xerographic developermixture of said first and said second component in the ratio range offrom 1 part of said third component to 3 to 10 parts of said firstcomponent whereby the dark area coverage of the developer is enhanced.

9. A developer in accordance With claim 8 in which the volumetricrelationship of said third component to said first component is about 5parts of said first component to 1 part of said third component.

10. A developer in accordance with claim 9 in which the third componentis of a size that will pass through a 270 mesh screen.

References Cited in the file of this patent UNITED STATES PATENTS2,297,691 Carlson Oct. 6, 1942 2,520,651 Oswald Aug. 29, 1950 2,618,551Walkup Nov. 18, 1952 2,618,552 Wise Nov. 18, 1952 2,638,416 Walkup May12, 1953 2,659,670 Copley Nov. 17, 1953

1. A TRIPARTITE DEVELOPER FOR ELECTROSTATIC LATENT IMAGES COMPRISING AFIRST COMPONENT CONSTITUTED BY MIXTURE COMPRISING A FIRST COMPONENTCONSTITUTED BY LOOSE, MOVABLE PARTICLES OF ELECTROSTATICALLY-ATTRACTABLETONER POWDER, A SECOND COMPONENT CONSTITUTED BY GRANULAR CARRIERMATERIAL COMPOSED OF LOOSE PARTICLES, EACH PARTICLE OF SAID SECONDCOMPONENT INCLUDING A CORE AND A COATING BONDED THERETO AND EACHPARTICLE OF SAID SECOND COMPONENT HAVING A TRIBOELECTRIC RELATIONSHIP OFOPPOSITE POLARITY TO THE TONER POWDER, AND A THIRD COMPONENT CONSTITUTEDBY A POWDER COMPOSED OF PARTICLES HAVING A RESISTIVITY IN POWDEER CFORMOF NO GREATER THAN LEAD ACETATE AND HAVING A TRIBOELECTRIC RELATIONSHIPWHICH IS STRONGLY NEGATIVE RELATIVE TO SAID CARRIER MATERIAL, SAID FIRSTCOPONENT COMPRISING PARTICLES IN THE SIZE RANGE OF FROM 0.1 TO 20MICRONS, SAID SECOND COMPONENT COMPRISING PARTICLES IN THE SIZE RANGE OFFROM 30 TO 200 MESH AND SAID THIRD COMPONENT BEING OF AN INTERMEDIATESIZE RANGE BETWEEN SAID FIRST AND SAID SECOND COMPONENTS, AND SAID THIRDCOMPONENT BEING ADDED TO A PROPER XEROGRAPHIC DEVELOPOER MIXTURE OF SAIDFIRST AND SAID SECOND COMPONENT IN THE RATIO RANGE OF FROM 1 PART OFSAID THIRD COMPONENT TO 3 TO 10 PARTS OF SAID FIRST COMPONENT.